This invention relates generally to liquid metal cooled nuclear reactors, particularly to a jacking apparatus for raising, lowering, and supporting an upper internals structure in a liquid metal reactor.
The design of the liquid metal nuclear reactor known as the Clinch River Breeder Reactor Plant incorporates a structure called an "upper internals structure" (UIS). The UIS has several functions: support and alignment of control rods above the reactor core fuel assemblies, support of instrumentation, core holddown, and coolant flow mixing. The UIS hangs from support columns supported from an "intermediate rotating plug" which is part of the reactor vessel head. The UIS is positioned at it lower end by radial keys which engage slots in a core barrel structure. The keys are important since these control UIS alignment and also oppose flow induced vibrations.
During reactor refueling the intermediate rotating plug must be rotated in order that a refueling machine may have access to all core fuel assemblies. This operation requires that the UIS be hoisted upward a distance (approximately 91/2 inches) sufficient to clear the UIS keys from the core barrel slots such that the UIS and intermediate plug are free to rotate. A jacking mechanism is used to accomplish the required hoisting and lowering of the UIS during refueling.
The support system for the UIS includes the support columns which pass the dead weight and other loads of the UIS through the jacks to the intermediate plug which transfers the load to the reactor head, which in turn is supported by structural features of a surrounding building.
During hoisting for refueling, it is considered essential that the UIS support system successfully react the UIS dead weight (approximately 100,000 lbs.), and key-slot misalignment loads (magnitude varies with degree of misalignment).
During reactor operation, it is considered essential that the UIS support system successfully react UIS dead weight, seismic loads (1,000,000 lbs.) and, additionally, possible forces, alternatingly upward and downward, on the UIS due to rapid vapor expansion from the liquid metal coolant in the reactor due to an accident. This force could be of the magnitude of 4,000,000 lbs. Since the accident involved is considered less likely during refueling, this large load capability may not be required during refueling.
During hoisting and lowering, small alignment errors between the UIS and the core barrel slots impose misalignment loads on the jacking mechanism in order to force the keys into or out of the slots. The misalignment load which the jacking mechanism must overcome is a function of the misalignment magnitude.
The UIS guides and aligns control rods to enable these to drop into fuel assembly control rod guide tubes during a reactor shutdown or "scram". The time required for the control rods to drop into the reactor core, called the "scram time", is an important safety parameter which is adversely affected by any misalignment of the UIS with the reactor core. Therefore, the jacking mechanism must be able to raise and lower the UIS for refueling operations with precision to prevent alignment errors, particularly those in the vertical direction not corrected by key insertion. The margin of error in lowering the UIS is considered to be 30 thousandths of an inch over 27 feet.
In summary, the jacking mechanism and support system for the UIS of a liquid metal reactor of the Clinch River variety must be capable of reacting very large forces, while also accomplishing precision UIS positioning.
It has been discovered that conventional jacking systems which can meet the indicated load requirements are massive in size and are not sufficiently precise. Precision jacks are not sufficiently strong. As an added complication, it has developed that the area in which the jacks must be located is crowded with other necessary equipment such that massive jacks cannot easily be accommodated.
Consequently, it is desired to provide a jacking mechanism capable of reacting large forces and of precision positioning of a UIS for a liquid metal reactor.